1. Field of the Invention
This invention relates to a method and apparatus for measuring the pure resistance of an in-vehicle battery mounted to supply electric power to a load in the vehicle.
2. Description of the Related Art
Generally, when a current is discharged from a battery, a drop in a terminal voltage of the battery occurs. The voltage drop, which is ascribable to the internal impedance (synthesized resistance) of the battery, consists of an IR loss (voltage drop due to pure resistance, i.e. ohmic resistance) due to the structure of the battery and a polarization resistance component (voltage drop due to activating polarization and concentration polarization). In the voltage-current (V-I) characteristic, the voltage drop ascribable to the IR loss does not vary as long as the battery maintains the same state, whereas the voltage drop ascribable to the polarization resistance component varies according to the value of the current and the time while the current flows. Therefore, if the various states of the battery are estimated from the V-I characteristic containing the polarization resistance component, an inaccurate result is estimated. This requires a technique of measuring only the pure resistance separated from the polarization resistance component.
The battery can be used repeatedly within a range of a charging capacitance by charging the battery so that the discharging current is compensated for. However, where an unexpected accident such as excessive discharging or shortage of electrolyte occurs, or a secular change of the battery occurs owing to the usage of for a long time, the dischargeable capacitance, which is electric energy that can be supplied to a load, falls abruptly. Therefore, in the state where dischargeable capacitance has fallen owing to the secular change, even when the discharging which exceeds the charging is generated for a very short time, there is a fear that an engine cannot be started again by energization of a starter motor after it has been stopped.
Additionally, in comparison between a new battery and that having suffered from the secular change, it is known that the latter has a larger resistance than the former. Therefore, in the routine inspection of a vehicle, it was proposed to measure the pure resistance as a standard of battery exchange. This is because knowing the pure resistance permits the degree of degradation to be determined taking the proportion of the pure resistance to the polarization resistance component. Knowing the degree of degradation can be also used to estimate the open circuit voltage of the battery.
Traditionally, the measuring apparatus that has been used to measure the pure resistance of the battery measures it when the battery is in a stationary state, or in an equilibrium state where there is no voltage rise or fall in the electrolyte owing to the polarization of the charging/discharging.
One example of this technique is to acquire the pure resistance from the relationship between the voltage and current that varies within a prescribed time of e.g. about 1 xcexcsec. in a state where either polarization due to charging or discharging is not accumulated by repeating the charging/discharging by application of the AC voltage at the frequency of 1 kHz-100 kHz to the battery. This uses the phenomenon as shown in FIG. 14 that after the discharging has been stopped, the voltage restores abruptly and thereafter restores gently. Now, it is assumed that the abrupt restoration within a prescribed time xcex94t is attributable to the pure resistance R and the subsequent gentle restoration is attributable to the other component (capacitance and inductance component) such as polarization exclusive of the pure resistance. On this assumption, the pure resistance is measured on the basis of the voltage and current within a short period during each application cycle of the AC at 1 kHz-100 kHz.
However, the battery mounted in a vehicle is stationary in only a limited case. Therefore, the above technique cannot be adopted while the vehicle is running.
In the above method, in order to collect the data of the voltage V and current I in a short time, the data sampling with a very short period and the subsequent A/D conversion within a prescribed time xcex94t are required. The above method has an advantage that it can be realized in a single measuring apparatus. However, it is difficult to implement the method in a vehicle. In addition, in order to acquire the xcex94V/xcex94I with great accuracy, the large values of xcex94V and xcex94I must be given. However, these large values can be measured in only a limited case in the vehicle. Further, any optional AC voltage cannot be applied to the battery while the vehicle is running. Accordingly, actually, the method described above cannot be realized to measure the pure resistance of the battery while the vehicle is running.
An object of this invention is to provide a method and apparatus for measuring the pure resistance of an in-vehicle battery which can measure the pure resistance while a vehicle is running.
In accordance with the first aspect of this invention is to provided a method for measuring a pure resistance of an in-vehicle battery which supplies electric power to a load of the vehicle comprising the steps of:
periodically measuring a terminal voltage and a discharging current of the battery when the discharging current, which increases monotonously to exceed a prescribed value and decreases monotonously from the maximum value to the prescribed value or lower, flows, thereby acquiring a first approximate equation of a voltage-current characteristic curve, indicative of correlation between the terminal voltage and the discharging current, for an increasing discharging current and a second approximate equation of the voltage-current characteristic for a decreasing discharging current;
setting a first point on the voltage-current characteristic curve represented by the first approximate equation and a second point on the voltage-current characteristic curve represent by the second approximate equation;
assuming a first assumed point and a second assumed point on the voltage-current curves represented by the first approximate equation and the second approximate equation, the first assumed point providing the same resistance as a second synthesized resistance composed of a pure resistance of the battery and a second polarization resistance component, which produces a second voltage drop when a second discharging current corresponding to the second point flows, and the second assumed point providing the same resistance as a first synthesized resistance composed of the pure resistance of the battery and a first polarization resistance component, which produces a first voltage drop when a first discharging current corresponding to the first point flows;
acquiring a first corrected gradient exclusive of a voltage drop due to the second polarization resistance component and a second corrected gradient exclusive of a voltage drop due to the first polarization resistance component, the first corrected gradient being acquired by correcting a first gradient of a first line connecting the second point and the first assumed point is corrected by a difference between the voltage drops due to the second polarization resistance component, which are produced by the second discharging current and the discharging current at the first assumed point, and the second corrected gradient being acquired by correcting a second gradient of a second line connecting the first point and the second assumed point by a difference between the voltage drops due to the first polarization resistance component, which are produced by the first discharging current and the discharging current at the second assumed point; and
acquiring an average gradient of the first corrected gradient and the second corrected gradient by averaging a sum of them so that the average gradient is measured as the pure resistance of the battery.
In this method, the terminal voltage and discharging current and discharging current are measured when electric power is supplied to a load in a normal using state of vehicle, and the data thus obtained have only to be processed to acquire the pure resistance of the battery.
Preferably, in this method, the first point and the second point are located on the voltage-current characteristic curves represented by the first approximate equation and second approximate equation within a range where there are terminal voltage and the discharging current measured to acquire these equations.
In this method, at least one data for acquiring the gradient is based on real data so that using point which is greatly deviated from point actually measured is avoided.
Preferably, the first point and the second point on the are located on the voltage-current characteristic curves represented by the first approximate equation and second approximate equation at a point corresponding to the maximum value of the discharging current of the battery measured to acquire these approximate equations.
In this configuration, at least one data for acquiring the gradient is based on real data so that using point which is greatly deviated from point actually measured is avoided. In addition, both points are set at the common points so that inclusion of an error can be suppressed.
Preferably, the first approximate equation and the approximate equation are quadratics. This permits the approximate equations more similar to the real data to be applied to acquire the gradients.
Preferably, newer sets of the terminal voltages and the discharging currents for a prescribed time are stored for collection in a memory. In this method, after it has been confirmed that the discharging current necessary acquire the first and second approximate equations has flowed, the first and second approximate equations can be acquired using the real data stored.
In accordance with the second aspect of this invention, the apparatus corresponding to the first aspect is provided, thereby giving the same effect. Namely, as shown in FIG. 1, there is provided an apparatus for measuring a pure resistance of an in-vehicle battery which supplies electric power to a load of the vehicle comprising:
means 23a-1 for periodically measuring a terminal voltage and a discharging current of the battery when the discharging current, which increases monotonously to exceed a prescribed value and decreases monotonously from the maximum value to the prescribed value or lower, flows;
means 23a-2 for acquiring a first approximate equation of a voltage-current characteristic curve, indicative of correlation between the terminal voltage and the discharging current, for an increasing discharging current and a second approximate equation of the voltage-current characteristic for a decreasing discharging current; and
means 23a-3 for setting a first point on the voltage-current characteristic curve represented by the first approximate equation and a second point on the voltage-current characteristic curve represent by the second approximate equation, assuming a first assumed point and a second assumed point on the voltage-current curves represented by the first approximate equation and the second approximate equation, the first assumed point providing the same resistance as a second synthesized resistance composed of a pure resistance of the battery and a second polarization resistance component, which produces a second voltage drop when a second discharging current corresponding to the second point flows, and the second assumed point providing the same resistance as a first synthesized resistance composed of the pure resistance of the battery and a first polarization resistance component, which produces a first voltage drop when a first discharging current corresponding to the first point flows, acquiring a first corrected gradient exclusive of a voltage drop due to the second polarization resistance component and a second corrected gradient exclusive of a voltage drop due to the first polarization resistance component, the first corrected gradient being acquired by correcting a first gradient of a first line connecting the second point and the first assumed point is corrected by a difference between the voltage drops due to the second polarization resistance component, which are produced by the second discharging current and the discharging current at the first assumed point, and the second corrected gradient being acquired by correcting a second gradient of a second line connecting the first point and the second assumed point by a difference between the voltage drops due to the first polarization resistance component, which are produced by the first discharging current and the discharging current at the second assumed point, and acquiring an average gradient of the first corrected gradient and the second corrected gradient by averaging a sum of them so that the average gradient is measured as the pure resistance of the battery.
In this apparatus, the terminal voltage and discharging current and discharging current are measured when electric power is supplied to a load in a normal using state of vehicle, and the data thus obtained have only to be processed to acquire the pure resistance of the battery.
Preferably, in this apparatus, the first point and the second point are located on the voltage-current characteristic curves represented by the first approximate equation and second approximate equation within a range where there are terminal voltage and the discharging current measured to acquire these equations.
In this apparatus, at least one data for acquiring the gradient is based on real data so that using point which is greatly deviated from point actually measured is avoided.
Preferably, in the apparatus, the first point and the second point on the are located on the voltage-current characteristic curves represented by the first approximate equation and second approximate equation at a point corresponding to the maximum value of the discharging current of the battery measured to acquire these approximate equations.
In this apparatus, at least one data for acquiring the gradient is based on real data so that using point which is greatly deviated from point actually measured is avoided. In addition, both points are set at the common points so that inclusion of an error can be suppressed.
Preferably, in the apparatus, the first approximate equation and the approximate equation are quadratics. This permits the approximate equations more similar to the real data to be applied to acquire the gradients.
Preferably, newer sets of the terminal voltages and the discharging currents for a prescribed time are stored for collection in a memory. In this apparatus, after it has been confirmed that the discharging current necessary acquire the first and second approximate equations has flowed, the first and second approximate equations can be acquired using the real data stored.
The above and other objects and features of this invention will be more apparent from the following description taken in conjunction with the accompanying drawings.